Titania-graphene anode electrode paper

ABSTRACT

A method for forming a nanocomposite material, the nanocomposite material formed thereby, and a battery made using the nanocomposite material. Metal oxide and graphene are placed in a solvent to form a suspension. The suspension is then applied to a current collector. The solvent is then evaporated to form a nanocomposite material. The nanocomposite material is then electrochemically cycled to form a nanocomposite material of at least one metal oxide in electrical communication with at least one graphene layer.

The invention was made with Government support under ContractDE-AC05-76RL0-1830, awarded by the U.S. Department of Energy. TheGovernment has certain rights in the invention.

TECHNICAL FIELD

This invention relates to nanocomposite materials with unique and usefulelectrochemical properties. These nanocomposite materials are formed ofgraphene and metal oxides. The invention has particular utility whenused in batteries and particularly in lithium ion batteries.

BACKGROUND OF THE INVENTION

There have been a number of examples of methods for formingnanomaterials using graphene and metal oxides to take advantage of theunique electrochemical properties of graphene. For example, U.S. patentapplication Ser. No. 12/462,857 filed Aug. 10, 2009 describesnanocomposite materials having at least two layers. Each layer consistsof one metal oxide bonded to at least one graphene layer. Thenanocomposite materials will typically have many alternating layers ofmetal oxides and graphene layers, bonded in a sandwich type constructionand will be incorporated into a electrochemical or energy storagedevice.

U.S. patent application Ser. No. 12/553,527 filed Sep. 3, 2009 describesa nanocomposite material formed of graphene and a mesoporous metal oxidehaving a demonstrated specific capacity of more than 200 F/g withparticular utility when employed in supercapacitor applications. Thesenanocomposite materials by forming a mixture of graphene, a surfactant,and a metal oxide precursor and then precipitating the metal oxideprecursor with the surfactant from the mixture to form a mesoporousmetal oxide. The mesoporous metal oxide is then deposited onto a surfaceof the graphene.

These and other prior art devices typically form the nanocompositematerials using a metal oxides in a salt form, such as lithium titanate(Li₄Ti₅O₁₂) as a precursor material. While this Li₄Ti₅O₁₂ material hasbeen shown to work well in these applications, it is expensive and thusmay not be suited for certain high volume applications.

Many of these metal oxides are widely known as inexpensive materials,but are also widely known as poor electrical conductors. For example,titania of the form TiO_(x) in its common forms of its anatase or rutileis widely known as an inexpensive material, but is also widely known asa poor electrical conductor. Therefore, those of ordinary skill in theart have not used these metal oxides, such as titania, as an anodematerial, or in applications where it would be a precursor to an anodematerial.

Accordingly, there exists a need for low cost metal oxides that can besuccessfully utilized as an anode material, or as a precursor to ananode material in applications where it would be combined with graphene.The present invention fulfills that need.

SUMMARY OF THE INVENTION

The present invention is therefore a method for forming a nanocompositematerial using low cost commodity chemicals as starting materials. Thepresent invention proceeds by first providing metal oxide and graphenein a solvent to form a suspension. The suspension is then applied to acurrent collector. The solvent is then evaporated to form ananocomposite material which has at least one metal oxide in electricalcommunication with at least one graphene layer. Preferably, the solventis an organic solvent, the metal oxide is titania, and the titania isprovided in a particle form wherein the particles have an averagediameter below 50 nm, and more preferably below 10 nm.

In one embodiment, the present invention is a method for forming ananocomposite material that includes the steps of providing metal oxideand graphene in a solvent to form a suspension. The suspension is thenapplied to a current collector. The solvent is then evaporated to forman anode. The anode is connected to a cathode having lithium ions and anelectrolyte to form a battery. The anode is then electrochemicallycycled to form a nanocomposite material of at least one metal oxide inelectrical communication with at least one graphene layer.

In another embodiment, the present invention is a nanocomposite materialformed by providing metal oxide and graphene in a solvent to form asuspension. The suspension is then applied to a current collector. Thesolvent is then evaporated to form an anode. The anode is connected to acathode having lithium ions and an electrolyte to form a battery. Theanode is then electrochemically cycled to form a nanocomposite materialof at least one metal oxide in electrical communication with at leastone graphene layer. Preferably, the solvent is an organic solvent, themetal oxide is titania, and the titania is provided in a particle formwherein the particles have an average diameter below 50 nm, and morepreferably below 10 nm. The nanocomposite material of the forgoingembodiment may further be formed by the steps of connecting the anode toa cathode having lithium ions and an electrolyte to form a battery andelectrochemically cycling the anode.

In another embodiment, the present invention is a battery formed byproviding metal oxide and graphene in a solvent to form a suspension.The suspension is then applied to a current collector. The solvent isthen evaporated to form an anode. A cathode and an electrolyte are thenprovided in electrical communication with the anode. The anode isconnected to a cathode having lithium ions and an electrolyte to form abattery. The anode is then electrochemically cycled to form ananocomposite material of at least one metal oxide in electricalcommunication with at least one graphene layer. Preferably, the solventis an organic solvent, the metal oxide is titania, and the titania isprovided in a particle form wherein the particles have an averagediameter below 50 nm, and more preferably below 10 nm.

BRIEF DESCRIPTION OF THE DRAWINGS

The following detailed description of the embodiments of the inventionwill be more readily understood when taken in conjunction with thefollowing drawing, wherein:

FIG. 1 is a graph of the cycling performance of TiO2/Graphene compositeelectrodes made by one embodiment of the present invention at various Crates using 2.6 micron graphene.

FIG. 2 is a graph of the charge/discharge voltage profiles ofTiO2/Graphene composite electrode at various C rates of between C/10 and10C using 2.6 micron graphene.

FIG. 3 is a graph of the cycling performance of TiO2/Graphene compositeelectrodes made by one embodiment of the present invention at various Crates using 11.6 micron graphene.

FIG. 4 is a graph of the charge/discharge voltage profiles ofTiO2/Graphene composite electrode at various C rates of between C/10 and10C using 11.6 micron graphene.

FIG. 5 is a graph of the cycling performance of TiO2/Graphene compositepaper electrodes prepared from an aqueous suspension.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

For the purposes of promoting an understanding of the principles of theinvention, reference will now be made to the embodiments illustrated inthe drawings and specific language will be used to describe the same. Itwill nevertheless be understood that no limitations of the inventivescope is thereby intended, as the scope of this invention should beevaluated with reference to the claims appended hereto. Alterations andfurther modifications in the illustrated devices, and such furtherapplications of the principles of the invention as illustrated hereinare contemplated as would normally occur to one skilled in the art towhich the invention relates.

Several experiments were conducted to demonstrate various alternativeembodiments of the present invention. In the first of these experiments,an aqueous method using a film type application was shown for formingtitania/graphene composites of 90/10 wt % and 72/28 wt % (P-25).

To prepare the 90:10 (wt %) titania:graphene suspensions, 23 mg graphene(Vorbeck Materials LLC) was dispersed in 50 ml H2O using 2.3 mg (10 wt%) CTAB (cetyl trimethylammonium bromide) surfactant and ultrasonicatedfor 15 minutes. For the 72:28 (wt %) titania:graphene suspensions, 77.8mg of graphene (Vorbeck Materials LLC) was dispersed in 50 ml H2O using7.78 mg CTAB (cetyl trimethylammonium bromide) surfactant andultrasonicated for 15 minutes. In a second container, 200 mg nanosizedtitania powder (Degussa P25) was dispersed in 50 ml H2O by stirring andultrasonic mixing for 10 minutes then slowly added to the graphenesuspension. The final suspension was then mixed for an additional 4hours. After mixing, the titania:graphene suspension is filtered, airdried then calcined at 400° C. in a H2/Ar atmosphere for 3 hours.

To prepare test electrodes, 0.5 ml of poly(vinylidene fluoride) (PVDF)binder dispersed in N-methylpyrrolidone (NMP) solution (0.5 g/20 ml) wasadded to 0.1125 g of the composite powder and homogenized for 10minutes. The final slurry was then coated on an Al current collectorusing a roll applicator to a thickness of approximately 60 microns.Slurry rheology was adjusted using NMP content and viscosities in therange of approximately 1000-5000 cps, which produced good quality films.After drying on a hot plate for 5 minutes, circular test electrodes weremade using a 9/16″ punch. The half-cells (2325 coin cell, NationalResearch Council, Canada) with polypropylene membrane separator(Celgard, Inc.), Li metal anode and reference in 1M LiPF6 in EC/DMC (1:1v/v) (ethyl carbonate/dimethyl carbonate) electrolyte were assembled ina glove box (Mbraun, Inc.) filled with ultra highly purity (UHP) argon.The electrochemical performance of the TiO2/graphene anode was thenevaluated using an Arbin Battery Tester BT-2000 (Arbin Inst., CollegeStation, Tex.) at room temperature. The half-cell was tested between 3Vand 1V vs. Li at various C rate current based on a theoretical capacityof 168 mAh/g (i.e., 1C=168 mAh·g−1) for anatase. In the next set ofthese experiments, an aqueous method using a tape application was shownfor forming titania/graphene composites of 90/10 wt % and 72/28 wt %(P-25).

To prepare the 90:10 (wt %) titania:graphene suspensions, 23 mg graphene(Vorbeck Materials LLC) was dispersed in 50 ml H2O using 2.3 mg (10 wt%) CTAB (cetyl trimethylammonium bromide) surfactant and ultrasonicatedfor 15 minutes. For the 72:28 (wt %) titania:graphene suspensions, 77.8mg of graphene (Vorbeck Materials LLC) was dispersed in 50 ml H2O using7.78 mg CTAB (cetyl trimethylammonium bromide) surfactant andultrasonicated for 15 minutes. In a second container, 200 mg nanosizedtitania powder (Degussa P25) was dispersed in 50 ml H2O by stirring andultrasonic mixing for 10 minutes then slowly added to the graphenesuspension. The final suspension was then mixed for an additional 4hours. After mixing, the titania:graphene suspension was filtered, airdried then calcined at 400° C. in a H2/Ar atmosphere for 3 hours.

For the preparation of P-25/graphene tapes (90 wt % P-25), 222.2 mg ofgraphene was dispersed in 250 mL H2O using 23 mg of CTAB surfactant andsonicated for 15 min. In a separate container, 2.0 g P-25 was dispersedin 100 mL water by sonication (10 min) The P-25 suspension was slowlyadded to the graphene dispersion upon stirring and stirred for 4 h. Theslurry was filtered, air-dried, and calcined at 400° C. in a H2/Ar for 3h.

To prepare P-25/graphene composite tapes, the calcined powder was firstdispersed in water and 7 wt % of PTFE suspension (65 wt % in water,Aldrich) was added upon stirring. After 3 additional hours stirring, themixture was filtered and dried at 90° C. for 30 min. The powder/PTFEgreen body was then calendared to the desired thickness (˜1-100 microns)using a three-roll mill.

Circular test electrodes were made using a 9/16″ punch and driedovernight at 110° C. in a vacuum oven. The half-cells (2325 coin cell,National Research Council, Canada) with polypropylene membrane separator(Celgard, Inc.), Li metal anode and reference in 1M LiPF6 in EC/DMC (1:1v/v) (ethyl carbonate/dimethyl carbonate) electrolyte were assembled ina glove box (Mbraun, Inc.) filled with ultra highly purity (UHP) argon.The electrochemical performance of the TiO2/graphene anode was thenevaluated using an Arbin Battery Tester BT-2000 (Arbin Inst., CollegeStation, Tex.) at room temperature. The half-cell was tested between 3Vand 1V vs. Li at various C rate current based on a theoretical capacityof 168 mAh/g (i.e., 1C=168 mAh·g−1) for anatase.

FIG. 5 shows the cycling performance of the titania/graphene compositeanodes produced using the aqueous film method. The anode shows a goodinitial capacity of approximately 120 mAh/g and approximately 20%capacity fade after 100 cycle at a C/5 rate.

In the next set of these experiments, a non-aqueous method also using afilm application was shown for forming titania/graphene composites of90/10 wt % and 72/28 wt % (P-25).

Nanosized titania powder (Degussa P25) and graphene (Vorbeck MaterialsLLC) were dispersed in NMP using ultrasonic mixing (30 min) in 90:10 and72:28 wt % ratios. Total solids loadings between approximately 3-12 wt %were typically used in preparing the initial suspensions. To theseslurries 10 wt % (relative to the solids content) of PVDF binder wasadded and the mixture stirred 5-16 h and homogenized if needed. Slurryrheology was adjusted using NMP and viscosities in the range ofapproximately 1000-5000 cps, which produced good quality films.

The final slurry was then coated on an Al current collector using a rollapplicator at a thickness of approximately 60 microns. After drying on ahot plate for 5 minutes, circular test electrodes were made using a9/16″ punch. The half-cells (2325 coin cell, National Research Council,Canada) with polypropylene membrane separator (Celgard, Inc.), Li metalanode and reference in 1M LiPF6 in EC/DMC (1:1 v/v) (ethylcarbonate/dimethyl carbonate) electrolyte were assembled in a glove box(Mbraun, Inc.) filled with ultra highly purity (UHP) argon. Theelectrochemical performance of the TiO2/graphene anode was thenevaluated using an Arbin Battery Tester BT-2000 (Arbin Inst., CollegeStation, Tex.) at room temperature. The half-cell was tested between 3Vand 1V vs. Li at various C rate current based on a theoretical capacityof 168 mAh/g (i.e., 1C=168 mAh·g−1) for anatase.

FIGS. 1-4 show the measured specific capacity at various C rates andvoltage profiles for the half cells prepared using this method. FIGS. 1and 2 were measured on titania/graphene composites made from graphenewith an average particle size of 2.6 μm. The samples had high initialcapacity (>170 mAh/g at C/10) and good rate capability (>100 mAh/g at2C). Less that 5% capacity fade occurred after 200 cycles at 1C, afterwhich the fading rate increased slightly. The sample retained a specificcapacity of approximately 100 mAhr/g after 400 hours cycling at 1C.

The data in FIGS. 3 and 4 was collected on titania/graphene compositesmade from graphene with an average particle size of 11.6 μm. Thesesamples had even better specific capacity, rate performance and cyclingstability than those prepared using the smaller (2.6 μm) grapheneparticles. Negligible fade occurs after 400 cycles at 1C in thesesamples. While the invention has been illustrated and described indetail in the drawings and foregoing description, the same is to beconsidered as illustrative and not restrictive in character. Onlycertain embodiments have been shown and described, and all changes,equivaλents, and modifications that come within the spirit of theinvention described herein are desired to be protected. Any experiments,experimental examples, or experimental results provided herein areintended to be illustrative of the present invention and should not beconsidered limiting or restrictive with regard to the invention scope.Further, any theory, mechanism of operation, proof, or finding statedherein is meant to further enhance understanding of the presentinvention and is not intended to limit the present invention in any wayto such theory, mechanism of operation, proof, or finding.

Thus, the specifics of this description and the attached drawings shouldnot be interpreted to limit the scope of this invention to the specificsthereof. Rather, the scope of this invention should be evaluated withreference to the claims appended hereto. In reading the claims it isintended that when words such as “a”, “an”, “at least one”, and “atleast a portion” are used there is no intention to limit the claims toonly one item unless specifically stated to the contrary in the claims.Further, when the language “at least a portion” and/or “a portion” isused, the claims may include a portion and/or the entire items unlessspecifically stated to the contrary. Likewise, where the term “input” or“output” is used in connection with an electric device or fluidprocessing unit, it should be understood to comprehend singular orplural and one or more signal channels or fluid lines as appropriate inthe context. Finally, all publications, patents, and patent applicationscited in this specification are herein incorporated by reference to theextent not inconsistent with the present disclosure as if each werespecifically and individually indicated to be incorporated by referenceand set forth in its entirety herein.

1. A battery formed by the process of: providing metal oxide andgraphene in a solvent to form a suspension, applying the suspension to acurrent collector, evaporating the solvent to form an anode, providing acathode and an electrolyte in electrical communication with the anode;electrochemically cycling the anode to form a nanocomposite materialcomprising at least one metal oxide in direct electrical communicationwith at least one graphene layer as part of the anode.
 2. The battery ofclaim 1 wherein the solvent is an organic solvent.
 3. The battery ofclaim 1 wherein the metal oxide is titania.
 4. The battery of claim 3wherein the titania is provided as particles having an average diameterbelow 50 nm.
 5. The battery of claim 3 wherein the titania is providedas particles having an average diameter below 10 nm.
 6. The battery ofclaim 1 wherein the solvent is NMP.
 7. The battery of claim 1 whereinthe graphene is dispersed in the suspension by a surfactant.
 8. Thebattery of claim 1 wherein graphene particles used to form thenanocomposite material had an average particle size of 2 to 3 μms.
 9. Abattery formed by the process of: dispersing graphene with a surfactantto form a first mixture; providing metal oxide in an aqueous solution toform a second mixture; mixing the first and second mixtures to form asuspension, applying the suspension to a current collector, evaporatingthe solvent to form an anode, providing a cathode and an electrolyte inelectrical communication with the anode; electrochemically cycling theanode to form a nanocomposite material comprising at least one metaloxide in direct electrical communication with at least one graphenelayer as part of the anode.
 10. The battery of claim 9 wherein the firstmixture further comprises an organic solvent.
 11. The battery of claim 9wherein the metal oxide is titania.
 12. The battery of claim 9 whereinthe titania is provided as particles having an average diameter below 50nm.
 13. The battery of claim 9 wherein the titania is provided asparticles having an average diameter below 10 nm.
 14. The battery ofclaim 9 wherein the current collector is aluminum.
 15. The battery ofclaim 9 wherein the graphene used for the first mixture has an averageparticle size of 11 μms.